The present invention relates to reinforcing fillers capable of reinforcing diene rubber compositions which can be used for the manufacture of tires or semi-finished products for tires, in particular treads for these tires. It relates more particularly to novel carbon blacks having a modified surface, and to processes for obtaining the same.
In order to reduce fuel consumption and the pollution emitted by motor vehicles, major attempts have been made by tire designers to obtain tires having all of the following characteristics: very low rolling resistance, improved grip both on dry ground and on wet or snow-covered ground, and very good wear resistance.
Numerous solutions have thus been proposed to lower the rolling resistance and to improve the grip of tires, but these have generally resulted in a very great decline in the wear resistance.
It is well known that the incorporation of conventional white fillers, such as silica (SiO2), alumina (Al2O3), titanium oxide (TiO2), chalk, talc, clays such as bentonite or kaolin for example, in rubber compositions used for the manufacture of tires and, in particular, of treads, results in a reduction in the rolling resistance and an improvement in grip to wet, snow-covered or icy ground. However, it also results in an unacceptable decline in the wear resistance because these conventional white fillers do not have sufficient reinforcement ability with respect to such rubber compositions. For this reason, these white fillers are generally referred to as non- reinforcing fillers, or alternatively inert fillers.
One effective solution to this problem was described in Patent Application EP-A-0 501 227, which discloses a special diene rubber composition, reinforced with a highly dispersible precipitated silica. This composition makes it possible to manufacture a tire having substantially improved rolling resistance, without affecting the other properties, in particular those of grip, endurance and, above all, wear resistance.
Since the publication of EP-A-0 501 227, interest in silica-reinforced compositions has been very largely revived. However, silicas are generally more difficult to disperse than carbon blacks. Furthermore, compositions filled with silica, compared with those filled with carbon black, are known to have the disadvantage of (1) being more difficult to work (i.e. suitability for transformation or xe2x80x9cprocessabilityxe2x80x9d) in the uncured state and (2) having a very high electrical resistance.
Fillers which are easier to disperse than silicas, such fillers being capable, of lowering the rolling resistance and improving the grip of the tires, like silicas, as well as providing a high level of reinforcement and, hence, of wear resistance, and also high electrical conductivity to the rubber compositions which they reinforce, are extremely desirable to tire manufacturers.
During the course of their, the Inventors have discovered a novel reinforcing filler, in this case a novel carbon black with modified surface (hereinafter referred to as xe2x80x9cmodified blackxe2x80x9d or xe2x80x9cmodified carbon blackxe2x80x9d) which unexpectedly makes it possible to satisfy these different contradictory requirements.
The invention relates to a process for coating at least in part, a carbon black with an adhering layer of aluminum oxide and/or hydroxide, this process comprising:
a) impregnating the carbon black with a colloidal suspension formed by hydrolysis of a solution of aluminum alkoxide in an alcoholic solvent;
b) removing the alcoholic solvent by evaporation; and
c) heat-treating the impregnated black so as to transform the aluminous layer present at its surface into an adhering layer of aluminum oxide and/or hydroxide.
The invention furthermore relates to any carbon black coated with an adhering layer of aluminum oxide and/or hydroxide, obtainable by the process of the invention.
One preferred embodiment of the invention relates to a modified carbon black of the type for reinforcing a tire, having the following characteristics:
(i) it is coated at least in part with a layer of aluminum oxide and/or hydroxide;
(ii) its specific BET surface area is between 30 and 400 m2/g;
(iii) its average particle size (by mass), dW, is between 20 and 400 nm;
(iv) its disagglomeration rate, xcex1, measured via an ultrasound disagglomeration test, at 10% power of a 600-watt ultrasonic probe, is greater than 1xc3x9710xe2x88x923 xcexcmxe2x88x921/s.
This reinforcing modified black for a tire is obtained by carrying out the process of the invention on a tire-grade starting carbon black. As a result of its specific combination of characteristics, notably its particular surface properties, the modified black has excellent dispersibility in diene rubber compositions intended for the manufacture of tires, and high reinforcement ability. It offers such compositions not only improved hysteresis and grip properties, but also high electrical conductivity.
Another subject of the invention is the use of the modified carbon black, as reinforcing filler in a diene rubber composition which can be used for manufacturing tires, particularly for treads for tires having low rolling resistance.
Another subject of the invention is a process for reinforcing a diene rubber composition which can be used for the manufacture of tires, in which the modified black is incorporated into this composition by mixing in an internal mixer, before the introduction of the vulcanization system.
Carbon blacks having a modified surface and coated with a siliceous layer have been described in recent patent applications, in particular as reinforcing filler for rubber compositions for tires (see, for example, EP-A-0 711 805, EP-A-0 799 854, EP-A-0799 867 and WO96/37547). Generally, these novel fillers and their potential for use in tires are still little-known to tire manufacturers. Compared with these carbon blacks coated with a siliceous layer, the modified blacks of the invention have the significant advantage that their aluminous surface layers, formed of aluminum oxides and/or hydroxides, which are more stable and chemically more reactive than silicon oxides, will consequently adhere better to the particles of black and will be more reactive than a siliceous layer. Furthermore, compared with silica, even a highly dispersible silica, the modified black of the invention has the advantage of being more readily dispersible in the rubber compositions and providing the latter with easier working in the uncured state.
Patent Application WO97/42256 describes carbon blacks treated directly in the synthesis reactor for the carbon black with various metal compounds in the form of oxides, hydroxides or carbides of different metals (for example aluminum, zinc, magnesium, calcium, titanium, vanadium, cobalt, nickel, zirconiwn, tin, antimony, chromium, neodymium, lead, tellurium, barium, caesium, iron and molybdenum) as potential fillers for rubber compositions. The blacks thus treated at very high temperature in the synthesis reactor consist of hybrid aggregates or particles having two phases, formed by a thorough mixture of carbon black and metal compound, with the metal compound being located both in the interior and close to the surface of the aggregates. It is specified that the content of metal atom may form 50% or even 99% of the mass of the final aggregate (% by mass). It will be understood that such hybrid fillers, if they were in some way freed of their metal compound fraction, for example by suitable chemical treatment, would have neither the morphology nor the properties of a conventional carbon black, but those of high-porosity carbon-containing residues. The treated carbon blacks described in WO97/42256 must not be confused with carbon blacks coated solely with a layer of metal compound which, once such a coating had been eliminated, resume their initial structures.